Sala A, Kundu M, Casella I, Engelhard A, Calabretta B, Grasso L, Paggi M G, Giordano A, Watson R J, Khalili K, Peschle C
Thomas Jefferson University, Kimmel Cancer Institute, Department of Microbiology-Immunology, Philadelphia, PA, USA.
Proc Natl Acad Sci U S A. 1997 Jan 21;94(2):532-6. doi: 10.1073/pnas.94.2.532.
B-MYB expression is associated with cell proliferation and recent studies have suggested that it promotes the S phase of mammalian cells. Based on its homology to the transcription factors c-MYB and A-MYB, B-MYB is thought to be involved in transcriptional regulation; however, its activity is not detectable in several cell lines. It was postulated that B-MYB function may depend on the presence of a cofactor, and recent studies suggested that B-MYB is phosphorylated specifically during S phase in murine fibroblasts. In this report we provide evidence that the product of the human B-myb gene can be activated in vivo by coexpression with cyclin A or cyclin E. Transfection studies showed that B-MYB was a weak transcriptional activator in SAOS-2 cells and was unable to promote their proliferation. In contrast, overexpression of both B-MYB and cyclin A or cyclin E caused a drastic increase in the number of SAOS-2 cells in S phase. Also, overexpression of cyclin A and cyclin E in SAOS-2 cells enhanced the ability of B-MYB, but not c-MYB, to transactivate various promoters, including the cdc2 promoter, the HIV-1-LTR, and the simian virus 40 minimal promoter. A direct role for cyclin-dependent activation of B-MYB was demonstrated using an in vitro transcription assay. These observations suggest that one mechanism by which cyclin A and E may promote the S phase is through modification and activation of B-MYB.
B-MYB的表达与细胞增殖相关，最近的研究表明它能促进哺乳动物细胞的S期。基于其与转录因子c-MYB和A-MYB的同源性，B-MYB被认为参与转录调控；然而，在几种细胞系中检测不到它的活性。据推测，B-MYB的功能可能依赖于一种辅因子的存在，最近的研究表明B-MYB在鼠成纤维细胞的S期会被特异性磷酸化。在本报告中，我们提供证据表明，人B-myb基因的产物可通过与细胞周期蛋白A或细胞周期蛋白E共表达在体内被激活。转染研究表明，B-MYB在SAOS-2细胞中是一种弱转录激活因子，无法促进其增殖。相反，B-MYB与细胞周期蛋白A或细胞周期蛋白E的过表达导致SAOS-2细胞S期数量急剧增加。此外，SAOS-2细胞中细胞周期蛋白A和细胞周期蛋白E的过表达增强了B-MYB而非c-MYB反式激活各种启动子的能力，包括cdc2启动子、HIV-1-LTR和猿猴病毒40最小启动子。使用体外转录试验证明了细胞周期蛋白依赖性激活B-MYB的直接作用。这些观察结果表明，细胞周期蛋白A和E促进S期的一种机制可能是通过修饰和激活B-MYB。
